A fastening component (hereinafter referred to as a threaded part), such as a screw, a bolt or a nut, which is essentially used in most industrial fields and is common in daily life is a mechanical element for fixedly fastening separate objects to each other. This is advantageous in that a fastening process is simpler than that of welding, riveting or bonding and objects joined by the threaded part may be separated again from each other if necessary.
A fastening method using the threaded part is advantageous in that a fastening structure is simple and a fastening force may be adjusted. However, this is problematic in that it may cause side effects when the threaded part is excessively strongly or weakly tightened.
For example, when the bolt is strongly tightened with torque exceeding a yield point, the bolt may be broken or lose resilience even if the bolt is not immediately broken, so that the bolt may not maintain designed fastening force for a long time. In contrast, if the bolt is tightened with force lower than optimal torque, the bolt may be gradually loosened by vibration or the like, or structural problems may occur because fastening force is small at a junction. For this reason, it is very important to apply prescribed torque when objects should be joined to each other.
Among control methods for adjusting torque applied to the threaded part, a torque-value control method using a torque wrench utilizes a linear relation between the applied torque and tightening force. Since the torque-value control method is affected by the frictional coefficient of a threaded part, axial force may be undesirably changed depending on a difference in frictional coefficient. That is, since the frictional coefficient of the threaded part varies depending on the machining precision of threads, surface-treatment specifications, the application of oil, etc., the frictional coefficient of the threaded part should be identified to obtain exact fastening torque.
Generally, in order to find a relation between tightening torque TT or loosening torque TL (hereinafter referred to as applied torque) and axial force Q when the threaded part is tightened or loosened, a frictional coefficient (tan ρ=μ) in threads and a frictional coefficient μn between the head of the bolt and a plate contacting therewith should be identified.
Furthermore, in order to identify the frictional coefficient μn, torque TB generated by the frictional force of threads and torque TW generated by the frictional force between the bolt head and the plate should be calculated. However, an apparatus for measuring the torque in general threaded parts has not yet been developed, and a relation TT−Q between the tightening torque TT and the axial force Q is merely experimentally calculated and applied.
In addition, in order to experimentally calculate the relation TT−Q between the tightening torque TT and the axial force Q, the axial force Q acting on the threaded part should be measured in real time while the threaded part is tightened with predetermined torque. To this end, a strain gauge should be attached to the threaded part or a special threaded part equipped with an axial force sensor should be used.
In order to attach the strain gauge to the threaded part, damage to the threaded part is inevitable. Thus, the structure of the threaded part is changed and thereby it is difficult to obtain a correct measured value. Since the special threaded part equipped with the axial force sensor is expensive and threaded parts vary in frictional coefficient, the test result obtained from the threaded part may be different from the test result of an actual threaded part.
Moreover, the experiment using the sensor may measure the axial force Q generated by the tightening torque TT of the threaded part, but may not measure the torque TB generated by friction of threads and the torque TW generated by friction between the head of the threaded part (or washer) and the plate. Consequently, it is impossible to calculate the frictional coefficient (tan ρ=μ) in the threads and the frictional coefficient μn between the head and the plate.
A bolt-loosening test apparatus, a so-called Junker tester may measure the axial force of the bolt using a load cell, but may not measure the torque TB generated by friction of the threads and the torque TW at each part generated by friction between the bolt head and the plate.
An apparatus and method for measuring a frictional coefficient disclosed in Korean Patent Laid-Open Publication 10-2010-0000565 (Title: apparatus and method for measuring frictional coefficient of threaded part) measures tightening torque and loosening torque by repeatedly tightening or loosening a threaded part using a drive motor and a reducer, indirectly calculates axial force using a theoretical equation based on a difference between two torques, and indirectly calculates a frictional coefficient using a theoretical equation based on the sum of two axial forces. However, the calculation using such an indirect method is problematic in that an error is large. Although the frictional coefficient (tan ρ=μ) of the threaded part is different from the frictional coefficient μn of the bolt head, the cited document performs measurement on the assumption that the frictional coefficients of the two parts are equal to each other, so that a calculated value contains many errors.